RE: Herbivore protection

[Stanley Friesen <sarima@ix.netcom.com>]

At 11:43 PM 5/31/97 -0400, John Bois <jbois@umd5.umd.edu> wrote:
>just maiasaurs, right?) were colonial nesters.  Why?  If there
>were T. rex and hadrosaurs in a given range, and the hadros want
>to swamp the predators, they don't need to gather together in
>colonies to do that.

That assumes that the colonial nest group did not gather from a larger area
than the predators could gather from, for one reason or another.  As the
passenger pigeon example shows, such mass nesting sites can involve
gathering all individuals from many thousands of square miles.  With few
exceptions, major predators are not able to match that feat.  (Humans being
the main exception).  And minor predators will usually be too small to
travel that far on foot.

Thus colonial nesting *can* improve the effectiveness of predator
satiation, by maximizing the localization to a greater extent than the
predators can match.

>hypothesize the following: all extant, colonial, vertebrate
>egg-layers are gregarious for one or more of three reasons, 1. nest
>defence, or mutual aid of some kind (crows, for example), 2. they
>are gathering on scarce low predator-density real estate (such as
>off-shore islands), or 3. scarce suitable nesting substrate
>(crocs, for example).  ...

Predator satiation is simply one, simple, form of nest protection. It is
quite likely that Maiasaura colonies also utilized other methods of nest
protection against such predators as were susceptible to them.
>
>     Another reason I believe predator satiation was not the
>prime adaptive value of colonial nesting in dinos is this: On the
>range (or wherever their regular seasonal foraging grounds were),
>hadrosaurs most likely had their "base number" of predators.  But
>at nesting time it is probable the predator pool was bigger. 
>Predators who, on the foraging grounds, could not exploit
>hadrosaurs (perhaps they couldn't find them as well as some
>others, or because they were marginally slower than some other
>predators), now could help themselves to easy nesting hadrosaur
>pickens.

True.  But concentration from a large enough area can exceed even this
increase.

>     Grunion, cited by Stan, and salmon spawning in the Chilkoot
>River, cited in a private post to me, are not analogous to
>colonial dinosaurs.  In both cases the species appear for a brief
>time.  And in both cases they are seeking to spawn in areas of
>low predator density, the high tide sand and the fresh water
>upper-reaches of the river respectively. 

High tide sands are low predator density??  Man, if that is so, then the
ocean must be saturated with predators to a degree unheard of on land!
Grunion are *heavily* predated on the beach - and not just by humans.
Gulls, terns, and foxes, and raccoons, and opossums all gather for the feast.

--------------
May the peace of God be with you.         sarima@ix.netcom.com
                                          sfriesen@netlock.com